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External bias supply for SMPS

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Selvam_Perumal

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I am looking for an external bias circuit for my SMPS whose Vcc is 12V to 15V@ 10mA.
But my input voltage is only 340V.
Zener regulator will result in the continuous power dissipation of nearly 5W.
To avoid the continuous power dissipation,Can you suggest the suitable circuit which will provide start up supply for my SM
PS Vcc.After that Vcc will be derived from the auxillary winding.
Once Vcc is derived from auxillary winding,start up regulator has to be disabled to avoid power dissipation.
 

A resistor and capacitor in series provide a handy delay circuit. Full supply voltage comes through at power-up. Then it drops as the capacitor charges. Power wastage is minimized. You can add a zener, or transistor, etc.
During power-off you need to discharge the capacitor in order to reset it. To do this add a high-ohm resistor across it, so capacitor voltage drops over a period of minutes or hours.
 

you can also use a 500-600V fet to provide initial aux supply, and use the real aux supply to turn it off, so that it does not dissipate any heat after start up ...
 

aswell as a startup capacitor, you could use a lnk302........or a startup capacitor that gets diode or'd out by the bias supply when it finally gets up and running.
 

If you use a small 800V fet to act as a start up source, be sure to put a fusible resistor in the Drain, and an ntc close to the device to allow turn off above 100 deg C - this will protect it against shorts on the Vcc, oh yes a 1A current limit is a good idea too, 1 ohm res in the source with a xtor to operate at 0.55V to pull down the gate ...

Nothing is worse than a touchy off-mains start up ckt that blows all the other cktry, some times a 5W 16V zener and fuse is a great idea to protect from such failures ...

Also a lot of ppl use a TiNY switch from PowerInt to give an isolated 12-15V off the rectified mains or boost bus (385 - 420VDC)
 
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Put a high value Resistor in series with a cap and zener. Tap the zener point to the Vcc of the IC. It should work flawlessly. Dont be too cautious on zener wattage. Use an IC which has low startup current


All the best!!!
 

Primary bias circuitry are a very common problem for SMPS design.

It can be divided in two broad categories:
-If all that you are going to power is the controller IC itself, usually a a startup circuit as described above, plus an auxiliary winding which will actually keep alive the controller during normal operation. All modern IC controllers feature very low startup current to minimize power dissipation.
-If you are going to power many other loads, like communication ICs, microprocessors, fans, etc, the best is to use either one of the small independent bias supply such as those offered by Power Integrations or ST Micro.
 

A depletion fet eliminates the need for a gate pull-up resistor which is a significant benefit when dealing with high voltage. This part is a depletion fet specifically marketed as a current regulator. With a zener this makes a nice bias supply that's inherently current limited and which can be easily turned off by grounding the gate.

A trick I designed but didn't try once, similar to what easy peasy suggested, is to use a PTC as the current setting resistor thermally coupled to the fet so that if it overheated it would shut off.

https://ixapps.ixys.com/datasheet/98704.pdf

Any similarly rated depletion mosfet would work identically (and there are a few similar but cheaper ones out there).
 

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